The invention relates to a threshold circuit for video signals, as obtained with a radar apparatus, for example.
Such a threshold circuit is widely known and is used for selecting strong video signals from other weaker video signals, while a large amount of clutter signals are eliminated. The term clutter signals is used to describe the collection of unwanted echo signals which smear the radar picture and render it impossible to identify wanted echo signals. Usually of interest are echo signals from moving targets; the remaining echo signals are therefore labelled as clutter when can be eliminated by means based on MTI and pulse Doppler techniques. This is not the case, however, with a harbor radar used to observe activities in the harbor area along and on the water. Here, not only of interest are echo signals from moving targets, but also from stationary targets; not only large structures, such as moored vessels and harbour buildings, but from smaller objects as well, such as buoys and beacons. In particular, the latter type of objects are difficult to identify on a radar screen amidst an abundance of clutter signals from rain and/or the water surface. In a harbor radar, the video signals from clutter cannot be eliminated by the abovementioned means, as this would also suppress wanted video signals from stationary targets. In the case in question, the clutter signals should be eliminated by means of a threshold circuit, where it is of great importance to select a suitable threshold level, as pointed out in Skolnik's Radar Handbook, Chapter 5.8. A too high threshold voltage constitutes the risk that video signals of, say, buoys and beacons are eliminated with the clutter signals, whereas a too low threshold voltage would tend to designate an excess of video signals as originating from genuine targets, thus causing an excessive false alarm rate. It is therefore important to select such a threshold value so as to result in a maximum number of detections of genuine targets with maximum clutter elimination and a minimum false alarm rate.
Since the clutter strength is not constant, but is dependent, on the one hand, upon factors usually varying slowly in value, such as wind-force, wind direction, harbor-traffic density, and length of the water waves, and on the other hand, upon factors which may vary rapidly in local places, such as the radar bearing relative to the direction of the waves, variations in wind-force and bow and stern waves, it is advantageous to adapt the threshold voltage continuously to the varied circumstances during the radar observations.